Plant-particle composites are well known in a variety of forms and can make use of comminuted plant material, especially wood chips, and various types of binders to produce rigid and coherent bodies for various purposes depending in large measure upon the density of the body. The binders may be phenol-formaldehyde resins, resorcinol resins or other thermosetting and thermohardening materials and the bodies may have strengths which range from load-bearing capacity to merely self-supporting strength and uses ranging from structural board or beam to insulating slab.
Naturally, within the realm of the prior art as within the scope of the present invention, other plant materials and other binders can be used and the products can be employed for other purposes as well.
While the art is aware of numerous methods of fabricating composite structures of the aforedescribed type, the present invention is concerned specifically with the cold-pressing or extrusion-pressing technique which is used for the production of beams and like relatively elongated bodies whose widths and thicknesses may be small fractions of their lengths.
In the production of such elongated bodies, the mixture of the plant particles, which generally have a "grain" or orientation as is the case with wood chips, and the binder in flowable form is metered into a chamber in which a piston plunger or ram is reciprocable to compress the mass and force it into and through a hardening passage having an internal cross section corresponding to the cross section of the body to be made, in which the mass is hardened, e.g. by the application of heat, high-frequency waves or the like, to form the continuous extruded body. The latter may then be cut into appropriate lengths.
Details of processes of this type can be found in German patent documents (Open Applications-Offenlegungsschriften) No. 22 53 121, 25 35 989 and 25 54 280.
In these systems, and in practice, the cold-extrusion pressing is effective with a relatively short stroke of the piston of about 1 to 15 mm, a relatively high frequency of strokes, for example 100 to 120 strokes per minute, and a very high compaction ratio of about 10:1, the latter representing the ratio of the final density (after compaction) to the original density (prior to compaction).
Similar parameters are used for the extrusion pressing of pressed board from wood chips.
When a product is made by extrusion in this fashion, it has an extremely high density and shows a very high specific gravity, so that it may be desirable to reduce the degree of compaction by, for example, the technique described in German patent document No. 25 35 989 whereby the wall sections of the hardening passage are slightly retractable during the press stroke. This reduces the friction during the compaction stroke. During the retraction of the piston, the extruded strand is somewhat relieved as well so that the individual chips or particles are not as adversely affected as would otherwise be the case by the high pulsating forces which are applied.
The structure of the strand fabricated in this manner is shown to consist predominantly of chips or particles whose grains or longitudinal directions run transversely to the direction of the extrusion, i.e. parallel to the compression front and the force-applying end of the piston or in curved layers which basically are transverse to the direction of compression and are convex in the direction in which the force is applied.
Extruded bodies of this structure can be fractured by forces transverse to the length of the body, i.e. by bending loads, parallel to the layers of the chips or particles.
Efforts have been made to revise the orientation of the chips of the extruded strand so that, for example, they will lie more or less perpendicular to the end faces of the latter before hardening and upon hardening will be locked into such orientations. Such approach is taught in German patent document No. 17 03 414 and utilizes a shaped cylinder in which a piston is reciprocable. Nevertheless, at the elevated compaction ratios, with the short strokes and the high repetition rate of the compaction strokes, problems are still encountered with the extruded body.
The fracture surfaces of the beams made by either of these aforedescribed techniques, resulting from the application of bending stress, have a cup-like or hemispherically concave shape.